Seasonal patterns of weight for age: Zimbabwe

A study has just been completed in Zimbabwe to examine seasonal patterns of malnutrition and to investigate whether these patterns vary within the country. A better understanding of seasonal variation should aid the interpretation of cross-sectional prevalence surveys carried out at different times of the year. This understanding may be critical in emergency situations where it is not always possible or appropriate to conduct nutritional surveys at the same time of the year. (Eds.)

The analysis was based on anthropometric data collected as part of a clinic-based growth monitoring programme that forms part of the national health information system (NHIS). Weight for age data of attending children under 5 years were analysed for the period 1988 - 1995.

No evidence of seasonal variation in underweight prevalence was found in the majority of districts studied (49 out of 60). In the remaining 11 districts, there was a small but significant increase in prevalence of underweight during January to March. This coincides with the period of food scarcity before harvest and when prevalence of diarrhoea and malaria are at their peaks. It is also the period of greatest workload for mothers. However, participation in growth monitoring also varies seasonally and could account for the increase observed.

No seasonal difference was observed on comparing subsistence with commercial agricultural districts.

The authors recognise a number of limitations when interpreting the NHIS data, e.g. non-systematic errors, such as miscalibration of weighing scales; incorrect age assessment or transcription and dataentry errors can occur. More importantly, there is a risk of systematic bias since the data are based purely on those who attend the clinics. It is possible that the observed patterns in prevalence of underweight are due to changes in the characteristics of attendees rather than seasonal variation of nutritional status of the population as a whole. It is possible for example, that underweight children are less likely to attend growth monitoring between April and June thereby accounting for the observed seasonal variability in percentage underweight in the eleven districts. Despite these limitations the authors draw the following conclusions from their data:

Seasonal influences on the estimated prevalence of underweight appear to be small and are unlikely to substantially affect estimation of underweight trends obtained through surveys.

Based on the authors' field experience, health services can expect to encounter slightly more cases of underweight children between January and March. Sudden increases in underweight are unlikely to occur.

Geographically this seasonal trend in underweight is just as common in rural commercial farming areas as in the subsistence based communal lands. However no seasonal pattern was observed in the major cities.

Food scarcity and a higher prevalence of diarrhoea, typically observed between January and March, is a greater risk factor for underweight than increased respiratory infections experienced during the drier, colder months.